Thinset, plaster, grout - okay to store in plastic?

I'm finishing up an apartment renovation and have a lot bags of plaster, thinset, grout, etc. These are all sold in paper bags or cartons and I'm wondering if there is a reason (besides economics) for this.

I figured that I'd save just a gallon or so of each for later patching/small projects in some old paint cans or sealable plastic pails but I want to make sure that they don't need to breath or something.